Double innovation during tunnel construction in Groningen

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The southern ring road in Groningen is being modernised so that it can handle the growing traffic flows around the city. This part of the ring road is being radically adapted. Traffic flows will soon be able to cross each other on three levels. This is possible due to an innovative tunnel construction technique with two intersecting tunnels stacked on top of each other.

Rijkswaterstaat (the Dutch Directorate General for Public Works and Water Management) is the client for this project. The Herepoort consortium is the contractor and Witteveen+Bos is responsible for the engineering. Julianaplein will be converted into an interchange with two intersecting tunnels. This solution was chosen instead of the variant with two flyovers. Underground tunnels make it possible to create a green junction that can be seamlessly integrated into the landscape and also limits traffic nuisance for the local community.

During the engineering phase, many different factors had to be taken into account. Not least due to the structure and the geological history. Settlement had to be factored into the design because pre-loading isn't possible at all the desired locations. The outer walls of the tunnels are fitted with tooth structures  to prevent settlement differences. This technique is normally more common in immersed tunnels but it has now also been used in these land-based tunnels as quality assurance.

Structural analysis in longitudinal and transverse direction

Due to the geological history and the upper layers after the construction of the N7 and A28, the subsoil in this location around the city of Groningen has a complex structure. As a result, the subsoil and the loads vary considerably over the length of the tunnels. This problem has been overcome with a cross-section at the centre of each tunnel section. In addition, every tunnel section end is also modelled.

The Plaxis calculation also showed displacements across the width of the tunnel floor. As a result, the structural analyses took both the longitudinal and transverse directions into account. A 3D bar model was chosen for the longitudinal analysis, where the bars are elastically supported and the section joint models are hinged. A seismic analysis was also conducted in the longitudinal and transverse directions.

Ingenious prefab reinforcement

As mentioned above, a complex and extensive reinforcement system is required to construct the intersecting and stacked tunnels. To meet this challenge, the engineers at Witteveen+Bos were able to make use of their extensive experience with 3D design techniques. The result is a smart prefab design. By braiding the reinforcements in series at a pre-assembly location, the structure can be built faster, more efficiently and with the best possible quality. Another very important advantage: the physical strain on the steel fixers is much lower. This is because they can now do their work standing up.

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